The present invention is in the field of glass-plastic composites and particularly relates to the manufacture of curved plastic elements for laminated glass-plastic lenses.
Glass-plastic lenses have been a subject of research because they combine the chemical durability of glass with the toughness, lightweight and tintability of plastics. An early example of such a composite is the laminated glass-plastic lens described by Rogers in U.S. Pat. No. 2,263,249, which combines a glass lens element with a curved plastic element composed of a light-polarizing polyvinyl alcohol plastic sheet, providing a durable polarizing lens. More recent examples of laminated glass-plastic lenses include the glass-plastic safety lens described by French et al. in U.S. Pat. No. 3,867,235, consisting of a conventional glass lens backed by a clear, tough, polymeric film.
A number of tough, optically clear thermoplastic films have been considered for composite fabrication, including, for example, cellulosic, polycarbonate, acrylic, polyvinyl alcohol and ionomer films. However, it appears that good optical performance in a laminated lens requires close curvature matching between glass and plastic elements. This matching is aided if preformed curved plastic elements are used in lamination.
Vacuum reforming processes have been used to reform relatively thick flat film, e.g., 30-mil thick cellulose acetate butyrate, into approximately spherically curved plastic elements for sunglass lenses and the like, but such processing is not adaptable to the reforming of thin (.ltoreq.15-mil thick) thermoplastic films. Moreover, curvatures obtained by vacuum reforming are not precise, and the optical quality of such elements has not been high. Finally, such techniques have not been extended to the production of curved non-spherical elements.
It is difficult to generate reforming surfaces sufficiently closely matched to non-spherically curved glass elements to reform plastic elements into curved elements which match those glass elements in curvature. Attempts to conform thermoplastic films to non-spherical curved surfaces using a pair of non-rigid (e.g., elastic) reforming members have not been successful. At the rather high pressures needed to achieve complete curvature conformity with some of the tougher films, elastic reforming members produce flow marks in the thermoplastic film which destroy optical quality.
It is a principal object of the present invention to provide a method for reforming thermoplastic film to provide curved plastic elements which substantially match the curvature of essentially any curved surface, but particularly surfaces of compound or aspheric curvature.
It is a further object of the invention to provide a method for reforming thermoplastic film to precise complex curvature which does not adversely affect the optical quality of the film.
Other objects and advantages of the invention will become apparent from the following description thereof.